1. Field of the Invention
The present invention relates to a semiconductor laser device and a manufacturing method thereof. More particularly, the present invention relates to a high-power group III-V compound semiconductor laser device capable of achieving improved electrical properties and optical efficiency, and a method for manufacturing the same.
2. Description of the Related Art
Recently, a semiconductor laser device has been used in numerous areas such as optical sensor, optical communication, optical pickup, and display. Also a variety of applications need a high-power laser device. Especially, AlGaAs-based or AlGaInP-based compound semiconductor laser device is utilized as an optical source of CD or DVD systems. In addition, there have been development and mass-production of a next-generation optical storage system using a blue-colored semiconductor laser device with 405 nm wavelength as an optical source.
In general, the semiconductor laser device includes upper and lower clad layers for supplying electric currents and an active layer formed therebetween to have induced emission of photons. The semiconductor laser device has the upper clad layer (e.g. p-type clad layer) formed as a ridge structure so that electric currents are supplied only through the ridge, thereby achieving improved injection efficiency of electric currents. WO 00/04615 discloses a group III-nitride semiconductor laser device having a ridge structure, and a method for manufacturing the same.
FIG. 1 is a sectional view of a conventional ridged AlGaInP-based semiconductor laser device. Referring to FIG. 1, the conventional ridged AlGaInP-based semiconductor laser device 10 includes an AlGaInP-based n-type clad layer 13, a p-type clad layer 17 and a p-type contact layer 22 sequentially stacked on a substrate 11 made of GaAs and etc. An upper part of the p-type clad layer 17 has a ridge structure, and a current blocking layer 24 is formed on both sides of the p-type clad layer 17. Also a p-electrode 26 is formed on the p-type contact layer 22. An active layer 15 may have a multiple quantum well structure, e.g. 2 optical guide layers and a multiple quantum well layer interposed there between.
In particular, in a high-power semiconductor laser device, the p-type clad layer 17 is doped with p-type impurities in a high concentration to prevent electrons injected from the n-type clad layer 13 to the active layer 15 from being leaked to the p-type clad layer 17 (to prevent degradation of heat characteristics). Zinc (Zn) or Magnesium (Mg) is mainly used as p-type impurities to dope the p-type clad layer 17.
Zn is easily doped in an AlGaInP-based semiconductor and has excellent electrical properties. But Zn diffuses easily when a doping concentration is high. Therefore Zn doped in the p-type clad layer 17 with a high concentration of 1018 cm−3 or more easily diffuses into the active layer 15, deteriorating optical properties and reliability of the laser device. Japanese Laid-Open Patent Application No. 2000-286507 teaches a method for stacking an undoped spacer layer between an active layer and a p-type clad layer to inhibit Zn diffusion into the active layer. However, a separate spacer layer stacked will complicate a process and increase costs thereof.
Compared to Zn, Mg allows high concentration without diffusion. Therefore, by using Mg as a dopant to dope the p-type clad layer, a device with superior heat characteristics is achievable. But, Mg doping causes time delay. Thus, even if Mg is used as a dopant in forming the p-type clad layer 17, the p-type clad layer is Mg-doped not immediately but after a certain time passes. Consequently, some areas of the p-type clad layer 17 adjacent to the active layer 15 is not Mg-doped, deteriorating electrical properties. Moreover, the lengths of Mg-undoped areas are not uniform, degrading reproducibility of device properties.